— Lanes eee 4 
THE CLOACAL MAMMALS. 233 
Mammals was made as early as 1816 by the eminent 
anatomist Blainville, who has already been mentioned, 
and who first clearly recognised the three natural main 
groups or sub-classes of Mammals, and distinguished them 
according to the formation of their generative organs as 
Ornithodelphia, Didelphia, and Monodelphia. As this 
division is now justly considered by all scientific zoologists 
to be the best, on account of solid foundation on the history 
of development, let us here keep to it also. 
The first sub-class consists of the Cloacal Animals, or 
Breastless animals, also called Forked animals (Monotrema, 
or Ornithodelphia). This class is now represented only by 
two species of living mammals, both of which are confined to 
Australia and the neighbouring island of Van Diemen’s land, 
namely, the well-known Water Duck-bill (Ornithorhynchus 
paradoxus) with the beak of a bird, and the less known 
Beaked Mole (Echidna hystrix), resembling a hedgehog. 
Both of these curious animals, which are classed in the 
order of Beaked Animals (Ornithostoma), are evidently the 
last surviving remnants of an animal group formerly rich 
in forms, which alone represented the Mammalia in the 
secondary epoch, and out of which the second sub-class, the 
Didelphia, developed later, probably in the Jurassic period. 
Unfortunately, we as yet do not know with certainty of 
any fossil remains of this most ancient primary group 
of Mammals, which we will call Primary Mammals (Pro- 
mammalia). Yet they possibly comprise the oldest of all 
the fossil Mammalia known, namely, the Microlestes antiquus, 
of which animals, however, we as yet only know some few 
small rolar teeth. These have been found in the upper- 
most strata of the Trias, in the Keuper, first in Ger- 
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